Auto-zeroed operational amplifiers are a class of analog circuits that combine analog and digital (switching) circuitry resulting in very low input-referred DC offset and noise. These operational amplifiers are often used in precision applications where high gain is necessary to resolve very small voltages. Examples include RTD, thermocouple, resistive current measurement and other sensing applications. The use of an auto-zeroed operational amplifier may relax the accuracy requirements of the A/D converter, saving cost. Today's auto-zeroed operational amplifiers bear little resemblance to the early chopping schemes that were used to reduce the average offsets. Those circuits were very simple using discrete amplifiers and switches to chop the amplifier's inputs and outputs using a clock. Heavy filtering was required to achieve low offset, and filter out the switching noise. Chopper amplifiers had a low frequency bandwidth, usually a few Hertz, limited by the large settling time constants.
Another scheme uses chopper stabilization of a wideband amplifier and a chopper amplifier. The chopper amplifier was used to “stabilize” or reduce the DC offset error of the wideband amplifier. Higher operating bandwidths were thereby achieved, but these chopped circuits had high output noise and required additional output signal filtering to be practical.
Early auto-zeroed amplifiers combined a wideband “main” amplifier and a single “nulling” amplifier. The single nulling amplifier has a sample and hold to correct its own offset, and to reduce the offset of the main amplifier. Early implementations required external capacitors and had sampling frequencies of a few hundred Hertz. Great improvements have been made in the state of the art over the years. Modern auto-zeroed operational amplifiers are now able to achieve DC offsets of a few microvolts with very low temperature drift.
However, due to the internal clock switching in the nulling amplifier, some switching noise will appear at the output. This will be most predominant around the sampling clock frequency. If this noise is not symmetric, i.e., generating substantially equal amounts of positive and negative glitches, an average DC offset may result in the system. Therefore, reduction of these glitches is essential for good DC performance and may require filtering. The filtering may be expensive depending upon how close the signal of interest is to the sampling frequency. Increasing the sampling rate is helpful to make filtering simpler and to increase the available bandwidth. Recent generations of auto-zeroed operational amplifiers have added clock spreading circuits to spread the switching noise over a wider range of frequencies allowing even higher useful bandwidths.